Polypropylene is a commercial polymer used for a variety of products such as packaging films and extruded and molded shapes. Polypropylene is typically deficient in impact resistance at temperatures of 0.degree. C. and below. Various polymers have been blended with polypropylene in order to increase the impact resistance. An Amoco Chemical Company Bulletin 12-35 titled "Improve the Impact Resistance of Thermoplastics with Amoco Polybutenes" discloses that the room temperature, 70.degree. F. Gardner impact of polypropylene is increased by the addition of 5 or 10 parts-per-hundred resin (phr) of L-14, H-100 and H-300 grades of Amoco polybutene. However, the low temperature, -30.degree. F., Gardner impact is not changed by the addition of polybutene. Combinations of polypropylene with other thermoplastics are discussed below.
U.S. Pat. No. 4,459,385 discloses extruded or molded shapes from compositions of ethylene-propylene copolymers and linear low density polyethylene (LLDPE) which possess extremely good impact resistance without excessive loss of stiffness. Examples N and O of this patent indicate that 15 or 20 percent LLDPE blended with mixtures of propylene homopolymer containing 5 to 11 percent of ethylene-propylene rubber does not provide any substantial increase in impact strength.
U.S. Pat. No. 4,500,681 discloses a thermoplastic elastomeric blend composition comprising about 20 to about 65 weight percent of a polyolefin component, about 20 to about 60 weight percent of an isobutylene-backbone elastomer and about 7 to about 40 weight percent of an ethylene copolymer.
U.S. Pat. No. 4,078,020 discloses a thermoplastic composition comprising polypropylene, polybutylene and 20 to 80 weight percent of an elastomeric material such as an ethylene-propylene-diolefin elastomer.
U.S. Pat. No. 4,311,807 discloses a masterbatch composition for the production of impact resistant polypropylene comprising about 25 to 70 weight percent of an uncured elastomer, about 30 to 75 weight percent of an isotactic butene-1 polymer and optionally about 10 to 40 weight percent of a co-modifier selected from the group consisting of high density polyethylene, propyleneethylene copolymer, polypropylene and mixtures thereof.
JP 60-203653 discloses a film made from a blend of 5 to 95 weight percent of linear low density polyethylene and 95 to 5 weight percent of polypropylene.
JP 59-049921 discloses a coating composition comprising 70 to 96 weight percent of crystalline polypropylene, 2 to 15 weight percent low density polyethylene and 2 to 15 weight percent amorphous ethylene-alpha-olefin copolymer.
JP 59-043043 discloses a resin composition used for film manufacture comprising 100 parts by weight of polypropylene, 3 to 10 parts by weight of linear low density polyethylene and 5 to 15 parts by weight of an ethylene alpha-olefin copolymerized elastomer.
JP 59-041342 discloses a resin molding composition comprising 95 to 60 parts by weight of linear low density polyethylene and 5 to 40 parts by weight of polypropylene.
EP 0 092 318 discloses a film-forming thermoplastic elastomer blend comprising a polyolefin component chosen from polypropylene homopolymer, polypropylene copolymer, polybutylene, high density polyethylene and mixtures of these polyolefin components; an elastomeric plasticiser comprising polyethylene or a copolymer of ethylene and an unsaturated ester of a lower carboxylic acid; an olefinic elastomer and a hydrocarbon oil.
GB 2152516 discloses a thermoplastic blend comprising 70 to 95 weight percent of linear low density polyethylene, 2 to 30 weight percent of polypropylene or propylene-ethylene copolymer and less than 2 weight percent of an ethylene-propylene-diene terpolymer or an ethylene-propylene rubber.
However, none of these patents disclose or suggest a composition comprising about 55 to about 80 weight percent polypropylene homopolymer, about 15 to about 30 weight percent linear low density polyethylene and 5 to about 15 weight percent polybutene which has improved impact resistance at ambient temperature and also at -40.degree. F.
An object of this invention is to provide a modified polymer composition with improved low temperature impact resistance.
Another object of this invention is to provide a composition comprising about 55 to about 80 weight percent polypropylene homopolymer, about 15 to about 30 weight percent linear low density polyethylene and about 5 to about 15 weight percent polybutene. Other objects appear hereinafter.
Film produced from polypropylene in general, has excellent physical properties at temperatures above 32.degree. F. However, at temperatures below about 32.degree. F., the polypropylene film tends to become brittle which is a disadvantage in a number of applications such as storage of food at temperatures between 10.degree. F. and 32.degree. F. When polypropylene film is used as a packaging material for such food application or general packaging, the film can fail if subjected to physical trauma at temperatures below 32.degree. F. The addition of LLDPE to polypropylene improves the impact resistance of the film composition at room temperature with little, if any, impact resistance improvement below 32.degree. F. However, incorporation of LLDPE and in accordance with this invention polybutene with polypropylene provides a significant enhancement in the impact strength of the composition in the form of film and molded parts at temperatures below 32.degree. F.